Ram disk on OS X

Create a 1.5G ram disk at /Volumes/ramdisk

$ diskutil erasevolume HFS+ "ramdisk" `hdiutil attach -nomount ram://3145728`

http://apple.stackexchange.com/questions/55794/why-do-mac-os-x-ramdisks-appear-to-be-limited-to-550mb-and-how-can-i-change-this